Neck pain can be caused by too much pillowing or not enough. Without the right amount of pillowing it is easy to strain your neck during sleep times. In this position, the head is lower than the neck, which can cause strain. Experiment with pillows to find a neutral position for your neck -- one in which you do not experience neck pain or strain.
Hage, M. (2005). The Back Pain Book. Atlanta, Ga. Peachtree Publishers.